1. Field of the Invention
The present invention relates to a liquid crystal display device including a dispersed type liquid crystal in which molecules of the liquid crystal are randomly distributed so that an incident light impinges on the molecules of the liquid crystal to scatter.
2. Description of the Related Art
Recently, liquid crystal display devices are applied to a variety of technical fields and the requirements for improving display quality, such as a large display surface, a high definition, and color display. Several types of liquid crystals are utilized in liquid crystal display devices. Twisted nemeatic liquid crystals have often been used, and in this case, molecules of the liquid crystal oriented in a constant direction relative to the transparent plates between which the liquid crystal is inserted. Alternatively, a liquid crystal of a dispersed type, such as a polymer dispersed liquid crystal has been developed and in this case, molecules of the liquid crystal are randomly distributed so that an incident light impinges on the molecules of the liquid crystal to scatter.
For example, FIG. 6 of the attached drawings shows a liquid crystal display device utilizing a twisted nematic liquid crystal. This liquid crystal display device includes a liquid crystal panel comprising a pair of opposed transparent plates 1 and 2, and a liquid crystal layer 3 sealignly inserted between the transparent plates 1 and 2. Electrodes and orientation layers (not shown) are provided on the inside surfaces of the transparent plates 1 and 2, respectively. The orientation layer of the upper transparent plate 1 is treated by rubbing in the direction, as shown by the arrow 1a, and the orientation layer of the lower transparent plate 2 is treated by rubbing in the direction, as shown by the arrow 2a. Accordingly, a portion of molecules of the liquid crystal near the upper transparent plate 1 oriented in the direction of the arrow 1a, and a portion of molecules of the liquid crystal near the lower transparent plate 1 oriented in the direction of the arrow 2a, with molecules of the liquid crystal between the upper and lower transparent plates 1 and 2 twisting by 90 degrees. An intermediate portion of molecules of the liquid crystal between the upper and lower transparent plates 1 and 2 is shown by the broken line 3a. Also, polarizers 4 and 5 are arranged, respectively, on the outside of the upper and lower transparent plates 1 and 2. The polarizers 4 and 5 have transmitting axes 4a and 5a of polarized light perpendicular to each other, in correspondence with the rubbing directions 1a and 1b of the upper and lower transparent plates 1 and 2.
In FIG. 6 , the line Z--Z is a perpendicular line standing at the center of the display surface. When this liquid crystal display device is arranged in an upright position in a vertical plane, the line Z--Z is a horizontal center line across the display surface and the line X--X is a vertical center line. In such a twisted nematic liquid crystal display device, a visual angular characteristic appears depending on a relationship between an angle by which a user sees the display and an orientation of the intermediate molecules 3a of the liquid crystal between the upper and lower transparent plates 1 and 2. In FIG. 6, the character .theta. shows a visual angle of depression and the character .phi. is a visual rotating angle. A user sees the liquid crystal display device from the front thereof when .theta. is zero, and sees the liquid crystal display device obliquely when .theta. is not zero. In the latter case, a user sees the upright liquid crystal display device from an upper position when .phi. is 90 degrees and sees the upright liquid crystal display device from a lower position when .phi. is 270 degrees.
FIGS. 7A to 7C show the twisted nematic liquid crystal display device to which lights are incident in the different directions A, B, and C. The arrow C shows a light incident perpendicular to the liquid crystal panel and the arrows A and B show lights incident obliquely to the liquid crystal panel. Also, FIG. 7A shows the liquid crystal display device when the applied voltage V is zero, FIG. 7B shows the liquid crystal display device when the applied voltage V is V.sub.S which is a relatively small value, and FIG. 7C shows the liquid crystal display device when the applied voltage V is V.sub.L which is a relatively large value. In the twisted nematic liquid crystal display device, an incident polarized light rotates 90 degrees in accordance with the twist of the liquid crystal, and the plane of vibration of the transmitting polarized light rotates 90 degrees relative to the plane of vibration of the incident polarized light. Accordingly, the polarized light transmitting the polarizer 5 (FIG. 6) can transmit the analyzer 4 by rotating 90 degrees. Also, as shown in FIGS. 7B and 7C, molecules 3 a of the liquid crystal rise as the applied voltage increases, so that an angle between the direction of the long axis of the molecules of the liquid crystal and the direction of the propagation of the light changes, causing a change in an optical effect of birefringence so that an intensity of transmitting light decreases. The optical effect of birefringence varies for the lights A, B, and C. For example, regarding the light C perpendicular to the liquid crystal panel, the optical effect by birefringence is "large" in FIG. 7A, "medium" in FIG. 7B, and "small" in FIG. 7C, with the intensity of the transmitting light decreasing in this order. Regarding the light A obliquely extending in the same sense as the tilt of the intermediate molecules 3a of the liquid crystal, the optical effect by birefringence is "medium" in FIG. 7A, "small" in FIG. 7B, and again "medium" in FIG. 7C. Also Regarding the light B obliquely extending in the opposite sense to the tilt of the intermediate molecules 3a of the liquid crystal, the optical effect by birefringence is "medium" in FIG. 7A, "large" in FIG. 7B, and again "medium" in FIG. 7C.
A visual angular characteristic of such a twisted nematic liquid crystal display device is typically summarized in FIG. 5. This visual angular characteristic is represented by a T-V relationship of a transmittance of a light through the liquid crystal 3 versus a voltage V applied to the liquid crystal 3. The curve F shows a visual angular characteristic when a user sees the liquid crystal panel from the front thereof, which corresponds to the case of the perpendicular incident light C of FIGS. 7A to 7C. The curve G shows a visual angular characteristic when a user sees the liquid crystal panel from a lower position, which corresponds to the case of the oblique incident light A of FIGS. 7A to 7C. The curve H shows a visual angular characteristic when a user sees the liquid crystal panel from an upper position, which corresponds to the case of the oblique incident light B of FIGS. 7A to 7C. In the visual angular characteristic G in the case of viewing the liquid crystal panel from a lower position, the transmitting light decreases with a slight increase of the voltage V and it is difficult to obtain a bright white display spot. Also, there is a portion M where brightness is reversed. Therefore, if a gradation display is attempted by controlling the voltage in correspondence with the brightness, a reversal of the resultant display brightness may result for example, a gray display spot may be produced at a position that is intended to produce a black display point, and reversely, a black display spot may be produced at a position that is intended to produce a gray display point. In reverse, in the visual angular characteristic H in the case of viewing the liquid crystal panel from an upper position, the reduction of the transmitting light is small even if the voltage V is increased to a greater value so that it is difficult to obtain a black display spot.
As described, there is the problem of a visual angular characteristic in the liquid crystal display device including an orderly oriented liquid crystal.
The liquid crystal display device including a liquid crystal of a dispersed type, such as a polymer dispersed liquid crystal, does not suffer from this problem in the twisted nematic liquid crystal display device because in the liquid crystal display device of a dispersed type, molecules of the liquid crystal are randomly distributed and the scattering of the incident light occurs in all directions, and accordingly a specific visual angular direction does not exist.
However, the dispersed type liquid crystal display device uses a medium in which the liquid crystal is dispersed. For example, in the polymer dispersed liquid crystal display device, the liquid crystal has the form of capsules that are dispersed in a polymer material. There is a problem of a leaking light in the dispersed liquid crystal display device, since the course of light is bent when the light propagates from the liquid crystal capsule to the polymer material, or vice versa, and the light may leak into adjacent display spots. If these adjacent display spots are black display spot, the leaking light graduates the quality of a black spot.
In addition, in the dispersed type liquid crystal display device it is desirable to produce a white display spot by scattering the impinging incident light when the voltage is not applied and produce a black display spot by causing molecules of the liquid crystal to rise relative to the transparent plates between which the liquid crystal is inserted when the voltage is not applied. However, there is a problem that, upon producing a white display spot by the scattering light, only a portion of the scattering light that has the same transmitting axis of polarized light as that of the polarizer on the light outlet side (analyzer) can transmit the latter and is utilized for producing a white display spot. But the remaining portion of the scattering light has a different transmitting axis of polarized light from that of the analyzer and thus cannot transmit the latter, and accordingly is not utilized. Therefore, the dispersed type liquid crystal display device suffers from a low utilization of light.